I would have to say Paper Mario. Not only did I play that game from sunrise to sunset the first time I got it, but I've played it over and over again. The fact that it was fun wasn't my only motivation though. My sister simply adored it, and wanted me to keep playing, hehe.
Other really addicting games were the pokemon series, the other Mario RPG's, Animal Crossing, (I played that game soo much.... and it takes at least a year to beat it, heh) SSBM, the Banjo-Kazooie series, and the Harvest Moon games. I just bought 'Harvest Moon: A Wonderful Life', so I'm playing it all the time, hehe. It really is amazing how addicting the game can be...